On the core switch, do you have a VI configured for each of those VLANs (205, 206)? This would be the default gateway for those clients and IP helper should be configured within those virtual interfaces in order for DHCP discover packets to be re-directed to the DHCP server accordingly.
May want to provide the rest of the switch config and/or, have you tested the DHCP server to ensure that it is leasing IPs on those scopes? A simple test could be connecting a PC directly to a switch port configured as access on vlans 205 and 206 and see if you get an IP, or creating a virtual interface on the access switch and set it to obtain an IP via DHCP.
Carlo, can you try the following please? I want to know if it helps at all. Can you configure the AP to bridge the VLAN instead of tunneling it to the controller? By this I mean that instead of having the AP create a tunnel to the controller, send the VLAN of the WLAN (VLAN 205) to the switch directly in a local break out mode and pass it along to your core switch. You may also want to enable dot1q encapsulation on the AP port and controller port and enable native VLAN for the management VLAN of the AP (if doing local break out) and controller.